Fernandes in Amorim’s system is much better in midfield than one of the 10’s as he has the game in front of him to pick passes and create, as one of the 10’s behind the striker he often has to drift out wide despite having zero pace and minimal dribbling ability.
For the 10’s to work behind the striker they need pace, dribbling ability, strong technical ability, positional sense, high stamina and work rate and an ability to run beyond the striker so outside of technical ability and positional sense Garnacho has all of those whilst only being 20 years old so with a strong coach like Amorim can be coached in those attributes currently lacking.
I think going forward a pairing of Fernandes and Ugarte in midfield with Mainoo, Collyer and another midfield all rounder coming in like Douglas Luiz covers everything in midfield and also is strong depth wise.
The perfect person to compare for the perfect 10 in an Amorim system is Ryan Giggs in his late 20’s as he could go out wide or play inside, had a fantastic work rate, brilliant dribbler, massively underrated passer, high technical ability and still had enough pace to get away from opposition players and whilst I’m not saying Garnacho will ever be to Giggs level he can be coached into being closer to that type of player.